At first, having the zombies crave brains seems like a snarky inversion of how Romero's zombies got killed by destroying their brains. Then you see zombie Trash stride out of the darkness, and you realize that if they'd eaten flesh like in the original Romero flicks, her body would've been all chewed up and that scene's twisted Fanservice would be ruined.
Also of note is that the human brain contains, among other things, endorphins! Serotonin, dopamine, and all sorts of feel-good chemicals that help us ignore how much our lives suck! So if it hurts to be dead, it makes sense why brains would be such an effective painkiller as opposed to, say,any other part of the human body.